Field of the Invention
The present invention relates generally to optical fiber devices and methods, and in particular, to improved high-birefringence hollow-core fibers and techniques for making same.
Background Art
Hollow-core fibers allow guidance of light almost entirely in a vacuum, or in a liquid or gas filling the hollow core. This capability opens up several possibilities, such as achieving extremely low optical nonlinearities in a potentially low-loss, bend-resistant fiber. The unique properties of hollow-core fibers are potentially useful in a number of different applications, including optical transmission, sensing, pulse compression, and the like.
For example, generally speaking, a hollow-core communications fiber would display significantly less nonlinearity than a silica-core fiber, dramatically changing system performance. For sensing applications, hollow-core fibers allow light to be guided in a gas or liquid sample introduced into a core of the fiber. This tends to maximize the interaction of light with a sample, which is very desirable for high sensitivity. In high-power pulsed laser systems, a hollow core allows very high peak power pulses to be delivered and compressed without experiencing nonlinearities or damaging the fiber.
In addition, hollow-core fibers are expected to have large polarization-mode impairments. This is significant because in many applications, such as communications, control of signal polarization is important. With respect to polarization-mode dispersion, generally speaking, it is desirable for a transmission fiber to have either a very low birefringence so that transmission is polarization independent, or a very high birefringence so that the polarization of transmitted signals can be controlled.
Generally speaking, it is difficult to make a hollow-core fiber effectively single mode and have a very low birefringence. Thus, birefringent hollow-core fibers that allow signal propagation in a well-maintained polarization are an attractive alternative. Similarly, in sensing systems and other applications, unwanted polarization coupling often contributes noise or impairment to the sensor output because of the uncertain division of light into two polarization modes. Thus, the polarization-maintaining property of a birefringent fiber can keep each signal in a definite polarization mode, despite perturbations that might otherwise induce polarization-coupling.
In one current design for birefringent hollow-core fibers, a core tube is added to an assembly of lattice capillary tubes, and a number of defect rods are positioned at the inner circumference of the core tube in a non-symmetrical pattern. Birefringence has been successfully demonstrated experimentally using such a design. However, the use of a core tube is not always desirable.
One reason is that the use of a core tube impacts the core-web thickness in the drawn fiber, which has an effect on performance. Different choices of core-web thickness may be desirable for achieving low loss, large bandwidth of low-loss windows, high birefringence, or other properties. Also, the positions of the defect rods are not fixed by a close-packed arrangement of elements. Thus, the defect rods are prone to becoming displaced from their desired positions, or even to becoming unintentionally detached. Irregularity in defect positions can lead to increased loss or drifting of the birefringent axis, which tends to degrade the polarization-maintaining performance of the fiber.